1. Field of the Invention
The present invention relates to an optical disc on which information is recorded in advance by means of pits having recessed and protruded shapes formed partially or entirely over a recording surface. More specifically, the present invention provides a technique for increasing recording capacity, preventing reduction in the capacity of recording main information by utilizing the increased capacity for recording additional information and enabling effective use of the recorded additional information. More specifically, the present invention relates to an optical disc of which pit depth represents information, an optical disc device for reproduction of the same, and the method of reproduction.
2. Description of the Background Art
A conventional optical disc is for binary recording, in which binary data correspond to presence/absence of pits.
When density of the disc is to be increased, the size of the pit is made smaller, and a laser beam spot for reading the same is also made smaller. Besides, multi-value recording in which one pit represents multi-valued data is also an effective means to attain higher density.
Japanese Patent Laying-Open No. 58-215735, for example, proposes an optical disc enabling recording of multi-valued data by changing pit depth in a plurality of stages so that quantity of light reflected therefrom changes in multiple steps. In this method, however, it is difficult to determine the level of the reflected light quantity, resulting in increased errors in the reproduced data. Japanese Patent Laying-Open No. 5-205276, alternatively, discloses a method of reproducing multi-valued recorded data by combining the level of the reflected light quantity and a push-pull signal level.
In an optical disc on which information is recorded by forming recessed and protruded pits in advance, as currently represented by a CD (Compact Disc) or a DVD (Digital Versatile Disc), the information is recorded in accordance with the method referred to as pit length recording, in which the information is mainly represented by presence/absence of a pit and the length of the pit.
FIGS. 1A to 1C are illustrations related to reproduction of a conventional optical disc on which information is recorded in accordance with the pit length recording method.
Referring to FIG. 1A, when a light beam 1101 directed from a pick up, not shown, onto the optical disc comes to a pit 131, the quantity of reflected light varies as shown in FIG. 1B because of interference and diffraction phenomenon of the light caused by the pit. When the reflected light is condensed on a photodetector, taken out as an electric signal and binarized by the comparison with a prescribed reference voltage, a reproduction data such as shown in FIG. 1C is obtained. By detecting presence/absence as well as the length of pit 131 based on the data, it becomes possible to reproduce the information. This is the principal of reproducing information from the conventional optical disc on which recording is done by the pit length recording method.
A CD and a DVD are much different in capacity. The difference derives from the difference in density of tracks, which are string of pits, as well as the difference in size of the pits formed on the discs. Further, optical wavelength and NA (Numerical Aperture) of the objective lens used are also different. More specifically, for a CD, NA of the objective lens is about 0.4 and the laser wavelength is about 780 nm to about 830 nm, whereas for a DVD, NA is 0.6 and the laser wavelength is 650 nm. The difference in the optical system results in the difference of the light beam.
The information recorded on the optical disc may be classified into the information eventually required by the user, that is, main information, and additional information for enabling efficient reproduction of the main information and for improving reliability of the main information.
Sound, image and characters may be the main information. The additional information may include index information, navigation information or the like for efficient reproduction, superimposed characters for a motion picture, subvoice, error correction code for improving reliability of the main information and address information representing a position on a disc. Recently, information for preventing unauthorized copying and a technique referred to as an electronic water mark, embedding information for protection within the main information have been studied, as techniques for preventing unauthorized reproduction of the information on the optical disc, and these may also be considered as additional information.
Existence of such additional information tends to increase the ratio of the additional information to the total capacity of the optical disc. With the limited recording capacity of the optical disc, increase of the additional information means decrease of the main information. To solve this problem, it is necessary to increase the recording capacity by improving recording density of the optical disc.
In order to increase the surface recording density of the optical disc, however, it is necessary to form smaller pits with high density, and to change optical system of a pickup for reproduction to generate smaller optical beam spot, from the reasons as described with reference to the difference in capacities of the CD and the DVD.
Japanese Patent Laying-Open No. 11-66607, for example, discloses a related technique. In this technique, as shown in FIG. 1 or FIG. 3 of this laid-open application, general main information is recorded as high density pits P0 for the CD, and large and low density pits P1 enclosing a number of pits P0 are recorded superposed.
The information of the high density pits (main information) is reproduced by an optical beam spot having the diameter of about 2 μm directed from a CD reading optical head H0 shown in FIG. 2 of this application, while the information of low density (additional information) is reproduced by an optical beam having the diameter of about 500 μm directed from a low density reading optical head H1 having a larger spot diameter.
In the method disclosed by Japanese Patent Laying-Open No. 5-205276, it is necessary to provide a beam spot for obtaining a push-pull signal, separate from the beam spot to obtain the reflected quantity of light, namely, two beam spots are necessary. When it is to be implemented by only one beam spot, it becomes necessary to offset the beam spot from the center of the track, or to cause relative positional deviation between the beam and the center of the track, by wobbling the track.
Such approach does not ensure stable tracking control, and therefore it is prone to deviation from the track or error in reproduction.
The technique disclosed by Japanese Patent Laying-Open No. 11-66607 requires two different types of optical heads (pickups). This leads to increased manufacturing cost and a larger scale device. Further, control of an optical head (pickup) for reproducing the additional information is also necessary. As the additional information is formed as large pits, the amount of recordable additional information is limited, and therefore decrease in the recording capacity of the main information caused by recording of the additional information cannot be avoided.
Various tracking servo techniques for positioning an optical beam on a pit string have been proposed for an optical disc reproducing device for reproducing an optical disc on which information is recorded by pits having recessed and protruded shapes formed in advance on a disc surface. Japanese Patent Laying-Open No. 58-150145 discloses an example.
FIG. 2 is a block diagram of the tracking servo in accordance with a phase difference (time difference) method disclosed by Japanese Patent Laying-Open No. 58-150145.
According to the phase difference (time difference) method, the reflected light beam from the optical disc is received by a photodetector having four elements divided along the radial direction of the optical disc and along the tangential direction. A sum signal of outputs of those of the photodetector elements which are positioned at opposing corners is found, and phase difference (time difference) of the sum signal is detected for tracking. In FIG. 2, the light beam reflected from the disc is condensed and directed to photodetector 22. Respective elements of the photodetector output signals corresponding to the incident light quantity. Sum amplifiers 23-1 and 23-2 calculate sum signals between portions a and c as well as b and d, which are positioned at opposing corners, of photodetector 22, and output the resulting sum signals to comparators (comparing circuits) 25-1 and 25-2. Comparators 25-1 and 25-2 compare the output signals from sum amplifiers 23-1 and 23-2 with reference signals +Ref1 and +Ref2, respectively, and provides as an output, a binary signal as a result of comparison.
As the reflected light beam has been diffracted by the pits, intensity distribution of the reflected light on the photodetector varies with time, dependent on the positional relation between the optical beam and each pit.
When the optical beam follows immediately above the pit string, for example, the sum signal of the outputs of elements (a+c) and the sum signal of the output from elements (b+d) which elements are at the opposing corners of photodetector 22 above the pits vary in the same manner. Therefore, output signals from comparators 25-1 and 25-2 also change in the similar manner at the same timing. When the optical beam follows a position deviated from just above the pit string, there would be phase difference (time difference) corresponding to the amount of deviation between the sum signal of the outputs (a+c) and the sum signal of the outputs (b+d), and either one of the sum signals change first, dependent on the direction of deviation.
Therefore, the phase difference (time difference) between the output signals from comparators 25-1 and 25-2 is detected by a phase comparing circuit 27, and a pulse corresponding to the phase difference (time difference) is provided. The pulse is passed through LPFs (Low Pass Filters) 28-1 and 28-2 to extract low frequency components only, and the difference therebetween is calculated by a differential amplifier 29. In this manner, a tracking signal indicative of the direction and amount of deviation between the optical beam and the pit string can be obtained.
Another example of the technique for obtaining the tracking servo signal is a push-pull method.
In the push-pull method, difference in the quantity of light on an inner periphery and on an outer periphery of a reflected light beam divided along the tangential direction is found, and the resulted difference is used as the tracking signal. FIG. 3 shows an example of a block configuration for generating the tracking servo signal in accordance with the push-pull method.
When the light beam is directed to a pit string, the reflected light is diffracted by the pits, dependent on the positional relation between the beam and the pits. In the push-pull method, the reflected light is divided into two and detected at the inner peripheral side and outer peripheral side of the optical disc, and a tracking servo signal is generated based on an average intensity.
Referring to FIG. 3, the reflected light beam is condensed onto a four-split photodetector 22, as in the phase difference (time difference) method described above. Sum amplifiers 23-1 and 23-2 provide sum signals of the outputs from elements on the inner peripheral side and sum signals of the outputs from the elements positioned on the outer peripheral side, not from the elements at opposing corners of the photodetector. The result of addition is output to differential amplifier 37. Differential amplifier 37 provides the difference between the two signals from sum amplifiers 23-1 and 23-2 to LPF 38. LPF 38 removes high frequency component of each pit from the difference, and extracts the low frequency component, that is, signal component which corresponds to average deviation between the light beam and the pit string. In principle, in the push-pull method, this signal is used as the tracking servo signal.
This method, however, has a problem that dependent on the depth of the pit formed on the optical disc, the polarity of the tracking servo signal may be inverted.